PROJECT SUMMARY/ABSTRACT Acute Respiratory Distress Syndrome is a major cause of mortality in critically ill patients. It also leads to prolonged mechanical ventilation and intensive care unit stays. Very few therapeutic strategies are available for patients with this syndrome. Interleukin-17A (IL-17A) is a pro-inflammatory protein that functions by recruiting other immune cells to sites of bacterial infection or inflammation. In murine models, IL-17A has been shown to mediate lung injury via neutrophil recruitment and increased epithelial barrier permeability. The cellular sources of IL-17A in murine studies are conflicting. Human studies show that IL-17A protein levels in the alveolar space are associated with the percentage of alveolar neutrophils in patients with ARDS but the cellular sources remain unknown. In Aim 1, this study proposes to use mass cytometry technology to identify the cellular source of IL-17A in alveolar samples from subjects with ARDS and compare the proportions of these cells to those in mechanically ventilated critically ill controls. Common genetic variation is one mechanism by which IL-17A responses may be variable between critically ill patients. In particular, IL-17A responses can be driven by agonists of the Toll-like receptor1/2 heterodimer in various cell populations. TLR1 genetic polymorphisms have been previously associated with hypersecretion of cytokines in whole blood and with organ failure in patients with sepsis. In Aim 2, this study proposes to identify whether common genetic variation in TLR1 modifies how IL-17A is produced in response to the TLR1/2 agonist Pam3CSK4 in critically ill subjects at risk for ARDS. We will also identify whether circulating cellular IL-17A production is associated with the development of ARDS in at risk patients. In a syndrome with few treatment options to improve patient outcomes, understanding how IL-17A is implicated in ARDS pathogenesis may provide a novel treatment strategy to dampen over-exuberant inflammation. The genetic nature of these studies may also further drive precision treatment strategies for subpopulations of patients with ARDS.